This invention relates to metal matrix composites.
Metal matrix composites are materials which include a metal matrix in combination with a secondary phase. The secondary phase acts as a filler or reinforcement in the composite. Metal matrix composites have different and often improved or more desirable properties as compared to their monolithic metal counterparts. For example, depending on the particular metal or metal alloy and secondary phases present in a composite material, as well as their respective ratios in the composite material, the composite material may have improved strength, stiffness, contact wear resistance, and elevated temperature strength, as compared to the corresponding monolithic metal or alloy. And, depending on the choice of reinforcing phase present in the composite, the metal matrix composites may be less costly and have lower density than their monolithic metal counterparts.
There are many potential applications for metal matrix composites. Significant applications of metal matrix composites are likely in automotive components, machine parts, and electronic packaging, as well as in specialized products based on unique combinations of properties.
Of particular interest are metal matrix composites comprising fly ash, because such composites are less expensive to prepare and exhibit improved properties with respect to their corresponding monolithic metal counterparts. Fly ash is an abundant by-product that results from the combustion of pulverized coal. In the past fly ash has been employed as a concrete admixture, as a soil stabilizer, as a filler for asphalt and structural materials, such as bricks. Fly ash is a low density particulate material, classed as precipitator fly ash (solid particles) and cenosphere fly ash (hollow generally spheroidal particles). Fly ash particles are predominantly micron sized, translucent particles which consist primarily of alumina, silica, iron oxides, lime and manganese.
A variety of methods for producing metal matrix composite materials have been developed. These methods include diffusion bonding, powder metallurgy, casting, high pressure infiltration of loose fly ash beds, spray codisposition and the like. For fly ash metal matrix composites in particular, stir casting and high pressure infiltration of loose fly ash beds have found use.
Although a variety of methods for metal matrix composite material production have been developed, these methods are not entirely satisfactory. Conventional techniques for combining metal matrix material and secondary phase material, for example, can result in uneven distribution of the secondary phase in the metal matrix, or inadequate levels of the secondary phase in the matrix; and it is generally difficult to control the amount of the secondary phase which is incorporated into the matrix. These problems have been particularly prevalent in attempts to produce fly ash metal matrix composites by conventional methods.
Rohatgi U.S. Pat. No. 5,228,494 describes making a metal matrix composite by stirring a secondary phase material such as fly ash or oil ash at high speed in a molten metal or metal alloy matrix material to make a slurry of metal and secondary phase material, and then casting the slurry in a suitable mold. The results of attempts using such techniques to obtain metal matrix composites containing more than about 30% by volume of fly ash are not fully satisfactory, because the viscosity of the slurry at higher volume fractions of fly ash becomes too great to permit molding, and because in slowly solidifying castings made by such techniques the fly ash tends to float and to become segregated in upper portions of the resulting cast composites.
Rohatgi U.S. Ser. No. 08/564,517, filed Nov. 29, 1995 and now issued as U.S. Pat. No. 5,711,362, describes combining a fly ash with a binder in an aqueous medium to form a slurry, drying the slurry to produce a solid, porous fly ash-binder preform, then infiltrating the solid preform with a molten metal matrix material, and cooling the metal-infiltrated preform to make a metal matrix composite. This method can yield aluminum-fly ash or lead-fly ash composites having the secondary phase uniformly dispersed within the composite at volume fractions greater than 30% and as high as 70%. However, interactions between metal and binder material may introduce undesirable characteristics in the resulting composite material.
U.S. Pat. No. 5,020,584 describes making metal matrix composite materials by infiltrating a preform of filler material, employing an infiltration enhancer material. Other U.S. patents of interest include U.S. Pat. No. 3,573,940; U.S. Pat. No. 3,585,155; U.S. Pat. No. 4,601,832; U.S. Pat. No. 4,888,054; and U.S. Pat. No. 4,936,270.